A unified approach to the specification of reflectance, in terms of both incident-and reflectedbeam geometry, is presented. Nomenclature to facilitate this approach is proposed. Under specified conditions-including uniform irradiance, a uniform, isotropic, plane surface, and allowance for edge effects due to sub-surface scattering-the geometrical reflecting properties of a reflecting surface are readily characterized or specified in terms of the bidirectional reflectancedistribution function (BRDF). The BRDF is denoted symbolically as fr:

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Wide generality for optical radiometry can be achieved by treating the basic radiometric quantities as field quantities. The treatment is that of classical ray optics, with emphasis on the geometrical relations involved. It is shown that radiance, defined as N ≡ ∂2P∂Ω cosθ∂A [W·cm−2·sr−1], the radiant flux or power per unit solid-angle-in-the-direction-of-a-ray per unit projected-area-perpendicular-to-the-ray, has the same value at any point along this ray within an isotropic medium, in the absence of losses by absorption, scattering, or reflection. More generally, the quantity N/n2 (where n is the index of refraction of the medium) in the direction of a ray is shown to be invariant along that ray, even across a smooth boundary between different lossless media. The usefulness of this invariant property of radiance is illustrated by examples of practical applications.

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